The present invention relates to so-called under-packing for rubber-coated fabrics mounted on offset printing press cylinders.
As known to one skilled in the art, an offset printing press cylinder consists of a hollow metal cylinder rotatable about its axis and carrying on its lateral surface a rubber-coated fabric sheet (the so-called printing blanket) provided on each of its opposing sides with a metal bar, normally of aluminium, of U cross-section to enable it to be mounted over the corresponding edge of the rubber-coated fabric and then fixed to it. The two metal bars serve to fix the rubber-coated fabric sheet to said cylinder. The rubber-coated fabric from which said sheet is obtained comprises two layers, of which one is the fabric and the other is rubber (normally caoutchouc).
For each model of offset printing press its manufacturer indicates the total thickness to which the blanket cylinder has to be covered, this covering consisting not only of the aforesaid rubber-coated fabric sheet (printing blanket) but also of so-called under-packing, traditionally formed from a supporting sheet of fairly thin rubber-coated fabric (the so-called under-blanket) and a certain number of graded paper sheets of various thicknesses which the user has to choose to obtain the necessary under-packing thickness. The supporting sheet (under-blanket) may also be not present, particularly in the more modern printing presses, so that in this case the under-packing is formed only from the graded paper sheets.
These graded paper sheets are stored in various boxes, each containing a multiplicity of graded paper sheets of the same thickness. All the graded paper sheets of the same thickness have the same colour, which is different from that of the graded paper sheets of other thicknesses in order to facilitate identification.
The user covers the under-packing (consisting of the under-blanket and the graded paper sheets) with the sheet of rubber-coated fabric (printing blanket), and fixes it to the impression cylinder by the said end metal bars. As will be apparent, the aforedescribed operation is lengthy and requires a certain care, all of which affects to a certain degree the operating costs of the printing press.
The object of the present invention is to substantially simplify and accelerate such an operation, with the advantage of substantially reducing said costs.
This object is attained by the universal under-packing of the present invention, comprising a determined number of superposed flexible sheets, characterised in that:
the total thickness of the universal under-packing is equal to a predetermined maximum thickness;
the thickness of the sheets of the universal under-packing is chosen such that by removing from the universal under-packing one or more graded sheets, the thickness which has to be obtained for any specific case can be equalled or sufficiently approximated to;
that face of each of the sheets of the universal under-packing which, when the under-packing is mounted on the impression cylinder, faces inwards, but with the exception of the face of the most inner graded sheet if the supporting sheet is not present, is treated with an sticker which, while maintaining the sheets united, enables them to be separated from each other.
The most inner sheet of the universal under-packing can be a traditional supporting sheet (an under-blanket), but can also be of other suitable synthetic material of adequate thickness.
By virtue of the present invention the user can be provided with a manufactured universal under-packing as a single unit, comprising (if requested) not only the graded sheets but also a supporting sheet. The user can then, according to the type of printing press available, detach from the universal under-packing those graded sheets which need to be removed in order to obtain an under-packing of the required thickness as determined for the specific case.
It should be noted that the component sheets of the universal under-packing of the present invention can still be of paper (but treated on one of their faces with sticker), although better results can be obtained by using sheets of a suitable flexible synthetic material (for example polyurethane sheets) which are normally more resistant while being detached (if care is not taken, the paper sheets can tear), so that they are sure to remain whole and reusable.
Except for the possible support sheet, the other (graded) sheets of the universal under-packing can be all of the same thickness, provided that this is sufficiently small to enable the predetermined required thickness to be approximated to the required extent, in order to ensure proper operation of the impression cylinder.
This obviously does not mean that graded sheets of different thicknesses cannot be used.
The constituent sheets of the universal under-packing can have a specific colour, indicative of a determined sheet thickness. Each sheet can also carry a number and/or letter indicating the specific thickness. In this manner it is extremely simple for the user to how many and which graded sheets to be removed from the universal under-packing to obtain the total thickness required to cover the cylinder of a determined offset printing press.